Female bumblebees visiting Chamaecrista latistipula flowers in the wild. The insect vibrates the internal parts of the flower to extract protein-rich pollen grains, which are taken to feed the colony’s larvae. – ANSELMO NOGUEIRA
September 27 () –
Bacteria that live in the soil and help roots fix nitrogen They can increase the reproductive capacity of certain plants.
It is the conclusion of an article published in the American Journal of Botany which describes a study of this mechanism in Chamaecrista latistipula, a legume belonging to the family Fabaceae, which includes beans and peas, reports FAPESP.
Soil bacteria improve the attractiveness of plant flowers to pollinators through a type of relationship known as mutualism, which is widespread in plants and animals. Microorganisms such as bacteria or fungi contribute to and benefit from mutualistic relationships with plants, through which both parties obtain more nutrients or reproduce more vigorously, for example.
In the case of C. latistipula, a shrub with a native range that spans Bolivia, Brazil, and northeastern Argentina, the soil it inhabits tends to be poor in nutrients and depends on a specific type of pollinator to reproduce.
“Their mutualistic relationship with nitrogen-fixing bacteria increases the supply of that nutrient to their roots in exchange for the sugar they feed on,” explained Anselmo Nogueira, professor at the Center for Natural and Human Sciences at the Federal University of ABC (CCNH). UFABC), in Brazil.
“The plant also has a mutualistic relationship with a specific type of pollinator. The pollen stored in the anthers of its flowers is only released when they are vibrated, mainly by being shaken by the females of some species of bumblebees of the genus Bombus“.
A greenhouse experiment conducted at the Plant-Animal Interaction Laboratory, which Nogueira directs, showed that these bacteria play an important role in making flowers attractive to bumblebees, especially for plants growing in nutrient-poor soils.
“We also observed a drastic effect that we did not expect. Since the association with bacteria is very costly for the plant, we assumed that in nitrogen-rich soil the plants would simply take nitrogen directly from the soil, but in our experiments, the nutrient-rich soil did not produced healthy plants with attractive flowers,” said Caroline Souza, first author of the paper.
The experiment was part of the project “Synergistic effect of multiple mutualists in plants: how bacteria, ants and bees contribute to the evolution of a hyperdiverse lineage of legumes”, which is coordinated by Nogueira.
In the experiment, The researchers monitored the growth of 60 C. latistipula plants from seed germination for 16 months. Half were grown in a soil composed mainly of sand (90%) with a thin layer of organic topsoil (10%) and a low concentration of nutrients, especially nitrogen. The other half was grown in soil rich in organic matter and supplemented with potassium nitrate, which releases nitrogen into the soil. In both cases, the acidity of the soil was controlled for six months to ensure that the pH was neutral and did not influence the root-bacteria interaction.
Before sowing the seeds, they were sterilized with alcohol, sodium hypochlorite and hydrogen peroxide to eliminate bacteria that could influence the results, and then rinsed in distilled water. The soil was sterilized at high temperature in an autoclave to eliminate microorganisms.
The two types of soil were then subjected to different treatments. A solution containing rhizobia (bacteria that fix nitrogen in the roots of plants) was added to half of the pots with nitrogen-poor sandy soil and to the other half with nitrogen-rich organic matter. The rest had no bacteria. The rhizobia used in the experiment were isolated directly from root nodules found on C. latistipula in nature.
In the nitrogen-poor sandy soil without added bacteria, the plants grew very little and had persistent yellowing leaves due to lack of nitrogen. Plants grown in nitrogen-poor sandy soil with added rhizobia developed satisfactorily.
“In the nitrogen-poor sandy soil with nitrogen-fixing bacteria, the plants were almost twice as tall and three times larger than those grown in nitrogen-rich soil with organic matter and rhizobia. On the other hand, the plants grown without rhizobia both in sandy soil and in soil rich in organic matter were lower and smaller than those grown with rhizobia,” said Nogueira.
The researchers analyzed the flowers using a surface spectrophotometer, which measures how light is reflected. “Based on the reflectance of the flowers measured in this way, We tested alterations in color contrasts perceptible to bumblebees in different soils with and without bacteria.“Souza said.
Significant differences were detected only in plants grown in sandy soil poor in nitrogen with rhizobia: their anthers presented a pattern considered particularly attractive to bumblebees, They perceive the color spectrum differently than humans.
“The anthers contain the pollen and can only be accessed by insects capable of making them vibrate, something that exotic species such as the European bee Apis mellifera, for example, cannot do,” explained Souza.
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